ALUDRA (Eta Canis Majoris). Northern winter skies may seem the
clearer, in part because of the brilliance of its stars. Canis Major (the Larger Dog) alone has five
stars of second magnitude or brighter, of which Aludra (magnitude
2.45) ranks fifth (nearly appropriate to its Greek designation
Eta). The name comes from an Arabic word that means "the
virginity," and alludes to the Arabic group called "the Virgins,"
which consists of the stars of the Dog's lower portions (and which
include Adhara and Wezen). Like its mate Wezen, Aludra is a
bright supergiant, but in blue-white class B (B5) it is much
hotter, and it may be the most luminous star in its constellation.
Its distance (from which luminosity is calculated), however,
presents a problem. Direct measures of parallax (the minute
positional shift caused by the orbiting Earth) by the Hipparcos
satellite gives 3200 light years, but with a very high potential
error. Some of "The Virgins", however, are more than just a chance
alignment; they form a loose, but real group, an "association"
called Collinder 121 that includes
Wezen, Aludra, Omicron-1 and
2
and Sigma, and which has an average distance of 1760 light years.
With this distance, Aludra shines with a total luminosity
(accounting for ultraviolet radiation from its 13,500 Kelvin
surface) of 66,000 Suns, a third again as great as Wezen's
luminosity. Temperature and luminosity conspire to give a radius
47 times that of the Sun, in good agreement
with the 43 solar radii gotten from the angular diameter.
Temperature and luminosity also tell of a star of about 15 solar
masses that is well along in its evolution. Born only 12 million
years ago, it has given up the fusion of hydrogen in its core and
is most likely making a very rapid transition to becoming a red
supergiant (though it is possible it may have already been one and
has now returned to the blue supergiant realm), after which it will
most likely explode as a supernova. As a dying, expanding star,
Aludra has a strong wind blowing from its surface at a speed of
some 500 kilometers per second. Now losing mass at a rate of
roughly a millionth of a solar mass a year (10 million times the
rate of the solar wind), it has already lost about a third the mass
of the whole Sun. The cloud it has generated around itself has
been detected in the infrared part of the spectrum via satellite.
Aludra has a seventh magnitude white "companion" about two minutes
of arc away that is easily visible in a small telescope. The
companionship, however, is not real, but just line of sight, as the
smaller star's class (it is an A dwarf like
Sirius) places it at a
distance of only 600 light years, much closer than Aludra. (Thanks
to Luis Lopes of Porto, Portugal, who suggested the star, and who
provided some basic information.)